Apparatus and method for determining the relative position and orientation of neurostimulation leads

ABSTRACT

Interelectrode impedance or electric field potential measurements are used to determine the relative orientation of one lead to other leads in the spinal column or other body/tissue location. Interelectrode impedance is determined by measuring impedance vectors. The value of the impedance vector is due primarily to the electrode-electrolyte interface, and the bulk impedance between the electrodes. The bulk impedance between the electrodes is, in turn, made up of (1) the impedance of the tissue adjacent to the electrodes, and (2) the impedance of the tissue between the electrodes. In one embodiment, the present invention makes both monopolar and bipolar impedance measurements, and then corrects the bipolar impedance measurements using the monopolar measurements to eliminate the effect of the impedance of the tissue adjacent the electrodes. The orientation and position of the leads may be inferred from the relative minima of the corrected bipolar impedance values. These corrected impedance values may also be mapped and stored to facilitate a comparison with subsequent corrected impedance measurement values. Such comparison allows a determination to be made as to whether the lead position and/or orientation has changed appreciably over time. In another embodiment, one or more electrodes are stimulated and the resulting electric field potential on the non-stimulated electrodes is measured. Such field potential measurements provide an indication of the relative orientation of the electrodes. Once known, the relative orientation may be used to track lead migration, to setup stimulation configurations and parameters for nominal stimulation and/or navigation. Also, such measurements allow automatic adjustment of stimulation energy to a previously-defined optimal potential field in the case of lead migration or postural changes.

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/338,331, filed Dec. 4, 2001, which applicationis incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to neurostimulation systems, such as aspinal cord stimulation (SCS) system, and more particularly to a methodfor determining the relative position and orientation of electrodes on aneurostimulation lead or leads used with such a system.

In SCS systems, positioning of the leads is critical to the success ofthe therapy. During surgery, the physician places the leads in a verycareful manner in order to locate the electrodes proximal to neuralelements that are the target of the stimulation. During and afterplacement, stimulation energy is delivered to verify that the leads areindeed stimulating the appropriate neural elements.

However, if the leads happen to shift position, the targeted neuralelements may no longer be appropriately stimulated. At best, this canrequire electrical reprogramming to restore therapy or, at worst,surgical revision, where a second surgery is required and the leads mustbe manually readjusted. In the first case, the physician may have only asuspicion that a lead has shifted position, based on patient reportingof paresthesia, which is not foolproof. Also, attempting to reprogramthe leads based on paresthesia locations can be challenging.

What is needed is a more objective technique for verifying the positionof the leads.

Prior art approaches for determining the lead position are disclosed inU.S. Pat. Nos. 4,486,835; 4,539,640; and 5,184,624, which patents areincorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing across-check technique for verifying the position of the electrodes ofthe implanted leads. A first technique involves the use ofinterelectrode impedance. A second technique involves measured fieldpotentials. Either technique advantageously allows the relativeorientation of one electrode on an implanted lead to other electrodes onthe implanted lead or adjacent implanted leads in the spinal column orother body/tissue location to be readily determined. Such techniques areuseful not only for reprogramming, but also to estimate if the shiftedorientation of the electrodes is sufficiently large so as to makeelectrical reprogramming a waste of time, thereby suggesting thatsurgery may need to be performed for repositioning.

At present, the correct lead position may only be determined by X-ray orfluoroscopy. Disadvantageously, X-ray and fluoroscopy require expensiveequipment, significant time, and appropriate medical facilities, most ofwhich are not readily available.

The general process for fitting a neurostimulation patient, i.e, aspinal cord stimulation patient, is described, e.g., in U.S. Pat. Nos.6,052,624; 6,393,325; in published international patent application WO02/09808 A1 (published 7 Feb. 2002); and in U.S. patent applications(assigned to the same assignee as the present application) Ser. No.09/626,010, filed Jul. 26, 2000; and Ser. No. 09/740,339, filed Dec. 18,2000, which patents, publication, and applications are incorporatedherein by reference. As indicated in those documents, prior to fitting apatient with the certain types of neurostimulation leads, the relativeorientation of the electrodes on the implanted leads should be known inorder to allow appropriate navigation of the stimulation energy. Atpresent, a determination of the relative orientation typically requiresthat a fluoroscope or X-ray image of the implanted leads be present atthe time of patient setup with the system programmer. Disadvantageously,however, such images may not always be available. Moreover, between thetime of implant and follow-up visits, the leads may have shifted and thefluoroscope image may no longer be valid. This can result in poorpatient outcomes due to inappropriate or unexpected stimulation effectsduring fitting.

Hence, it is seen that there is a need for the cross-check techniquesprovided by the present invention, which techniques can be used toverify the position of the leads at numerous times during the lifetimeof the implanted leads, e.g., during initial implantation andprogramming, during followup visits, throughout the trial period, andduring subsequent reprogramming sessions.

BRIEF DESCRIPTION OF THE DRAWINGS

The aspects, features and advantages of the present invention will bemore apparent from the following more particular description thereof,presented in conjunction with the following drawings wherein:

FIG. 1 illustrates a neurostimulation system wherein two leads, eachhaving eight in-line electrodes thereon, are positioned side-by-side,and wherein each lead is connected to an implantable pulse generator(IPG), which IPG is, in turn, coupled to an external programmer;

FIG. 2 shows a functional block diagram of an IPG that uses multipleprogrammable current sources to activate selected electrodes of theneurostimulation leads;

FIG. 3 shows a functional block diagram of an IPG that uses multipleprogrammable voltage sources to activate selected electrodes of theneurostimulation leads;

FIG. 4 is a table that contains impedance vector and distance impedancedata in accordance with one embodiment of the invention;

FIG. 5 illustrates representative relative electrode orientation in apatient having dual quadrapolar leads (two side-by-side leads, eachhaving four in-line electrodes thereon);

FIG. 6 is an impedance map that illustrates application of oneembodiment of the invention to the electrode orientation shown in FIG.5;

FIG. 7 depicts a representative fluoroscopic image of dual quadrapolarleads in a patient;

FIG. 8 illustrates, in accordance with another embodiment of theinvention, the measured electrode potential of non-activated electrodeson the dual quadrapolar lead of FIG. 7 when the activated electrode isactivated through monopolar stimulation;

FIG. 9 illustrates the measured electrode potential of non-activatedelectrodes on the dual quadrapolar lead of FIG. 7 when the activatedelectrodes are activated through tripolar stimulation; and

FIG. 10 is a flowchart that highlights the main steps used with oneembodiment of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claim(s).

The present invention uses: (1) interelectrode impedance (one techniqueor embodiment) or (2) measured field potentials (another technique orembodiment) to determine the relative orientation of one electrode on animplanted lead to other electrodes on the implanted lead or adjacentimplanted leads in the spinal column or other body/tissue location.

Before describing the two techniques, either of which may be used, itwill be helpful to first briefly provide an overview of a representativeneurostimulation system of the type with which the present invention maybe used. A representative neurostimulation system is illustrated inFIG. 1. Such system may include a first implantable lead 20 and a secondimplantable lead 30. Each lead includes a series of in-line electrodesthereon. For the example shown in FIG. 1, the first lead 20 containseight in-line electrodes E1, E2, E3, . . . E8. The second lead 30 alsocontains eight in-line electrodes E9, E10, E11, . . . E16.

Each of the electrodes of each lead 20 or 30 are electrically connectedthrough respective wires, embedded or carried within a body of the lead,to an implantable pulse generator (IPG) 40. The wires connected to theelectrodes E1, E2, E3, . . . E8 of lead 20, for example, may becharacterized as a bundle of wires 22 that are electrically connectedwith the IPG 40. Similarly, the wires connected to the electrodes E9,E10, E11, . . . E16 of lead 30 may be characterized as a bundle of wires32 that are electrically connected with the IPG 40. Through these wires,carried within the respective leads 20 or 30, the IPG is able to directelectrical stimulation to selected electrodes of each lead.

When a given electrode is selected to receive an electrical stimulus, itis (for purposes of the present invention) said to be “activated”. Whenan electrode is not selected to receive an electrical stimulus, it issaid to be “non-activated”. Electrical stimulation must always occurbetween two or more electrodes (so that the electrical currentassociated with the stimulus has a path from the IPG to the tissue to bestimulated, and a return path from the tissue to the IPG). The case ofthe IPG may function, in some modes of operation, as a return electrodeE_(R). Monopolar stimulation occurs when a selected one of theelectrodes of one of the leads 20 or 30 is activated along with thereturn electrode E_(R). Bipolar stimulation occurs when two of theelectrodes of the leads 20 or 30 are activated, e.g., when electrode E3of lead 20 is activated as an anode at the same time that electrode E11of lead 30 is activated as a cathode. Tripolar stimulation occurs whenthree of the electrodes of the leads 20 or 30 are activated, e.g., whenelectrodes E4 and E5 of lead 20 are activated as an anode at the sametime that electrode E13 of lead 30 is activated as a cathode. Ingeneral, multipolar stimulation occurs when multiple electrodes of theleads 20 or 30 are activated.

The IPG 40 is typically programmed, or controlled, through the use of anexternal (non-implanted) programmer 60. The external programmer 60 iscoupled to the IPG 40 through a suitable communications link,represented in FIG. 1 by the wavy arrow 50. Such link 50 passes throughthe skin 18 of the patient. Representative links that may be used tocouple the programmer 60 with the IPG 40 include a radio frequency (RF)link, an inductive link, an optical link, or a magnetic link. Theprogrammer 60, or other similar external device, may also be used tocouple power into the IPG for the purpose of operating the IPG orcharging a replenishable power source, e.g., a rechargeable battery,within the IPG. Once the IPG 40 has been programmed, and its powersource has been fully charged or replenished, it may operate asprogrammed without the need for the external programmer 60 to bepresent.

Turning next to FIG. 2, there is shown a representative functional blockdiagram of one type of IPG 40 that may be used with a neurostimulationsystem. As seen in FIG. 2, the IPG 40 therein depicted is made up of amultiplicity of dual current sources 42. Each dual current source 42includes a positive current source, i.e., a current source that canfunction as an anode to “source” current to a load, and a current sourcethat can function as a cathode to “sink” current from a load through thesame node. The “load” is the tissue that resides between the two or moreactivated electrodes, and includes the wire (or other conductiveelement) and a coupling capacitor C that connects the electrode to thecommon node of the dual current source.

Thus, for example, and as depicted in FIG. 2, a first dual currentsource connected to electrode E1 of a first lead through a couplingcapacitor C, may be programmed to produce a current of +I1 or −I1through electrode E1, depending upon whether such dual current source isconfigured to operate as a cathode or an anode, when such first dualcurrent source is turned on or enabled. Similarly, a second currentsource, connected to electrode E2, when turned on or enabled, may beprogrammed to produce a current of +I2 or −I2 through electrode E2. In asimilar manner, a third current source, when enabled, may be programmedto produce a current of +I3 or −I3 through electrode E3. An nth currentsource, where n represents the number of electrodes on the first lead,is similarly connected to electrode En, and may be programmed to producea current of +In or −In through electrode En when turned on or enabled.

If a second lead, also having n electrodes, is positioned adjacent thefirst lead, each electrode is similarly connected to a dual currentsource. For example, electrode E(n+1) is connected to a dual currentsource that produces a current of +I(n+1) or −I(n+1) through electrodeE(n+1) when such (n+1)th current source is enabled. In like manner, allof the electrodes of the second lead are connected to respective dualcurrent sources. There are thus 2n dual current sources that arerespectively connected to each of the 2n electrodes of the first andsecond leads (n electrodes on each lead). Alternative embodiments (notshown) may employ less than 2n dual current sources connected to 2nelectrodes through a suitable multiplexer circuit.

A programmable current control circuit 44 is also provided within theIPG 40 that controls, i.e., turns on or enables, at specified times, aselected current source to operate as either a cathode or an anode tosource or sink a current having a desired amplitude. The control circuit44 also disables, or turns off, selected current sources, as controlledby programmed control data received from the external programmer, orotherwise resident within the IPG. The control circuit 44 furtherincludes the ability to measure the electrode voltage, E_(V1), EV_(V2),E_(V3), . . . E_(Vn), . . . E_(V(2n)), appearing at the output of eachdual current source 42, whether the electrode is activated ornon-activated. This effectively allows the electrode voltage, orelectric field at the electrode, to be measured, which in turnfacilitates impedance or field potential measurements to be made, whichmeasurements are used in carrying out various steps of the invention asdescribed below.

Thus, in operation, and as illustrated in FIG. 2, current controlcircuit 44 may turn on current sources +I1 and +I2 at the same time,i.e., during a time period T1, that current source −I(n+2) is turned on.All other current sources are turned off, or disabled, during the timeT1. Such action causes electrodes E1 and E2 to be activated as anodes atthe same time that electrode E(n+2) is activated as a cathode. That is,a current +I1 is “sourced” from electrode E1 and a current +I2 is“sourced” from electrode E2 at the same time that a current −I(n+2) is“sunk” into electrode E(n+2). The amplitudes of the currents +I1 and +I2may be any programmed values, and the amplitude of the current −I(n+2)should be equal to −(I1+I2). That is, the current that is sourced isequal to the current that is sunk.

After the time period T1, it is common to switch the polarities of theelectrodes during a second time period T2. During T2, the electrodes E1and E2 are activated as cathodes, so that they both sink current, andelectrode E(n+2) is activated as an anode, so that it sourches a currentequal in amplitude to the current that is sunk by electrodes E1 and E2.In this manner, a biphasic stimulation pulse 46 is produced that ischaracterized by a first pulse (during time period T1) of one polarity,followed by a second pulse immediately or shortly thereafter (duringtime period T2) of the opposite polarity. The electrical chargeassociated with the first pulse is made so that it is equal to thecharge associated with the second pulse, thereby maintaining chargebalance during the stimulation. (Maintaining charge balance whenstimulating living tissue is generally considered an important componentof a stimulation regime.) Charge balance is commonly achieved in abiphasic pulse 46 by making the amplitude of the first pulse during timeT1 equal to the amplitude of the second pulse during time period T2,where T1 equals T2. However, charge balance may also be achieved usingother combinations of pulse duration and amplitude, e.g., by making theamplitude of the second pulse equal to ½ the amplitude of the firstpulse, while making the time period T2 equal to twice the time periodT1.

Next, with respect to FIG. 3, a functional block diagram of another typeof IPG 40′ that may be used in a neurostimulation system is shown. TheIPG 40′ shown in FIG. 3, includes a multiplicity of dual voltage sources42′, each being connected to one of the electrodes E1, E2, E3, . . . En,of a first lead, or to one of the electrodes E(n+1), E(n+2), . . .E(2n), of a second lead. Each dual voltage source 42′ applies aprogrammed voltage, of one polarity or another, to its respectiveelectrode, when enabled or turned on. For the configuration shown inFIG. 3, a separate dual voltage source 42′ is connected to eachelectrode node through a coupling capacitor C. (Other embodiments, notshown, may use one or two or more voltage sources that are selectivelyconnected to each electrode node through a multiplexer circuit.)

The control circuit 44′, or other circuitry within the IPG 40′, furtherincludes the ability to measure the electrode current, E_(I1), E_(I2),E_(I3), . . . E_(In), . . . E_(I(2n)), flowing to or from its respectiveelectrode, whether the electrode is activated or non-activated, and theelectrode voltage, E_(V1), E_(V2), E_(V3), . . . E_(Vn), . . .E_(V(2n)), appearing at the output of each non-activated dual voltagesource 42′ These measurements facilitate impedance and electric fieldmeasurements or calculations to be made, which measurements are used incarrying out various steps of the invention as described below.

A programmable voltage control circuit 44′ controls each of the dualvoltage sources 42′, specifying the amplitude, polarity, and duration ofthe voltage that is applied to its respective terminal. Typically,stimulation is achieved by applying a biphasic stimulation pulse 46′ tothe selected electrodes, wherein a voltage of a first polarity andamplitude is applied during time period T3, followed by a voltage of theappositive polarity and amplitude during time period T4. The biphasicstimulation pulse 46′ may be applied between any two or more electrodes.

It should be noted that the functional block diagrams of FIGS. 2 and 3are functional diagrams only, and are not intended to be limiting. Thoseof skill in the art, given the descriptions presented herein, should beable to readily fashion numerous types of IPG circuits, or equivalentcircuits, that carry out the functions indicated and described, whichfunctions include not only producing a stimulus current or voltage onselected groups of electrodes, but also the ability to measure thevoltage, or the current, flowing through an activated or non-activatedelectrode. Such measurements allow impedance to be determined (used witha first embodiment of the invention) or allow electric field potentialsto be measured (used with a second embodiment of the invention), asdescribed in more detail below. A preferred IPG is described ininternational patent application WO 02/09808 A1 (published 7 Feb. 2002);and in U.S. patent application Ser. No. 09/626,010, filed Jul. 26, 2000,which publication and application have been previously referenced andare incorporated herein by reference.

With the descriptions of FIGS. 1–3 thus providing background informationrelative to a neurostimulation system, the present invention will nextbe described. As has been indicated, the present invention addresses theproblem of determining the relative position between electrodes once theleads on which the electrodes are carried have been implanted. Thepresent invention uses: (1) interelectrode impedance (one technique orembodiment) or (2) measured field potentials (another technique orembodiment) to determine the relative orientation of one electrode on animplanted lead to other electrodes on the implanted lead or adjacentimplanted leads in the spinal column or other body/tissue location.

First, the interelectrode impedance technique of the invention will beexplained in connection with FIGS. 4–6. The interelectrode impedancetechnique is performed by measuring impedance vectors. A vector isdefined as an impedance value measured between two electrodes in thebody. The value of the impedance vector is due primarily to two physicalentities:

-   -   (1) the electrode-electrolyte interface; and    -   (2) the bulk impedance between the electrodes.

The impedance tomography technique of the present invention relies uponthe latter of the above two physical entities, i.e., upon the bulkimpedance between the electrodes.

The bulk impedance portion of the impedance vector may be further brokenup into two contributing factors: (a) the impedance of the tissueadjacent to the electrodes; and (b) the impedance of the tissue betweenthe electrodes.

The first factor (part a) makes up the majority of the measurement, dueto the higher and non-uniform current densities near the electrodesurface. However, the second factor (part b), where the current densityis more uniform, has a roughly linear relationship to distance betweenthe two electrodes, due to the definition of resistance. Resistance, R,is defined asR=(resistivity)×(distance)/cross-sectional area.

The second factor (part b) is used by the interelectrode impedancetechnique embodiment of the invention to determine the relative spacingbetween electrodes and to determine the relative orientation of theleads.

By way of example, one first-order, simple embodiment of the inventionis as follows:

If two multipolar leads are placed in the spinal column, see FIG. 5,each having four electrodes (the electrodes of one lead being designatedas e1, e2, e3, and e4; and the electrodes of the other lead beingdesignated as E5, E6, E7 and E8), their relative orientation may beinferred by making the following measurements:

-   -   1. Monopolar impedances for all electrodes; and    -   2. Bipolar impedances between each electrode on opposing leads.

The monopolar impedances are used to “correct” the bipolar impedancesfor the first factor of bulk impedance, the strongly-weighted impedancenear the electrode. The corrected bipolar impedances are then used todevelop an impedance “map” between the electrodes. This map reveals therelative orientation of the leads.

To illustrate, a sample correction formula is as follows:(distance  between  two  electrodes  e1  &  e2) ∝ (measured  bipolar  impedance  between  two  electrodes  e1  &  e2) + (2 * offset) − (monopolar  Z  for  electrode  e1) − (monopolar  Z  for  electrode  e2),where offset=an estimate of the impedance in the monopolar impedancemeasurement that is NOT due to the tissue near the electrode.

After the bipolar impedances are corrected by the above formula, therelative orientation of the leads may be inferred by the relative minimaof the impedance values. Where the corrected bipolar impedance betweentwo electrodes is a minimum relative to other electrodes on an opposingarray, those electrodes are relatively adjacent. This information maythen be loaded into a programmer, which can then provide a graphicdisplay of the assumed relative lead positions. Such data and/or displaymight then be compared with previously measured or entered and storedgraphics, indicating earlier orientations. Such comparison can thus helpthe physician/clinician to track the lead orientation to determineappropriate programming, reprogramming, or need for surgical revision.

Also, for some programming systems, the present invention may be used toautomatically setup the appropriate navigation tables for steeringmultiple lead systems.

FIG. 4 illustrates data showing this simple embodiment applied to datafrom a patient with dual quadrapolar leads, which leads are oriented asdepicted in FIG. 5. FIG. 6 shows the impedance map resulting from themeasurements of FIG. 4. It can be seen that the impedance maps (FIG.6)correlate well to the orientation of the leads (FIG. 5).

The simple interelectrode impedance technique described above may beenhanced by making more accurate corrections using the appropriate fieldequations to calculate the monopolar and bipolar impedance of theelectrodes. Also, other geometric methods may be employed using theimproved “distance impedance” values to improve the mapping of theelectrode orientations.

Next, an alternative technique or embodiment for determining relativeelectrode positions for multipolar leads of a neurostimulation systemwill be described. Such alternative technique utilizes electric fieldmeasurements of the implanted electrodes, and more particularly,electric field measurements on non-active electrodes caused byactivation of other electrodes. In a preferred embodiment of thisalternative embodiment, a constant current is sourced (anodes) and sunk(cathodes) from a predefined combination of electrodes. Such electrodesthus comprise the activated electrodes. Then, the resulting potentialsare measured at all other electrodes (those not involved in sourcing orsinking current), i.e., the non-activated electrodes. From thesemeasured potentials, the relative orientation of the electrodes, and theleads on which the electrodes are carried, may be determined.Advantageously, the use of field potentials represents an improvementover the use of impedance measurements, since the measured potentialvalues are less subject to the confounding effects of the tissueimpedance very close to the source/sink electrodes.

By way of example of this electric field potential measurementtechnique, consider FIGS. 7, 8 and 9. FIG. 7 represents the relativeposition of dual quadrapolar leads 21 and 31 after being implanted in apatient, as obtained using a fluoroscopic imaging device. In manyinstances, the necessary imaging equipment needed to obtain afluoroscopic image, such as is shown in FIG. 7, is not readilyavailable. Advantageously, the present electrical field potentialmeasurement technique represents an alternative approach to obtainingrelative electrode position information rather than using an expensiveand cumbersome imaging device.

Two combinations of anodes/cathodes are used to deliver current to theleads of the dual quadrapolar leads 21 and 31. The first technique ismonopolar (current delivered or sourced from one electrode—thecathode—and sunk to the return electrode E_(R)—the anode). Thus, foreach active monopolar combination, there are seven non-active electrodeson which the electric field may be measured. The second technique isflanked tripolar stimulation (current delivered between two anodes andone cathode, with the cathode being flanked on each side by an anode).

In both the monopolar stimulation and the tripolar stimulation, aconstant current is delivered to each electrode implanted in thepatient's body while the electric field potential is measured on allother electrodes NOT involved in sinking/sourcing current. The constantcurrent may be set to a subperception level, or to another suitablelevel that is comfortable for the patient.

The electric field potentials for the monopolar stimulation are plottedon the same chart in FIG. 8. The vertical axis is millivolts. As seen inFIG. 8, the electrodes closest to the source electrode have a highpotential (note: all plots in FIG. 8 and FIG. 9 are “negative”, i.e.,more negative potentials results in more positive measured values, asshown in the plots). Thus, for example, consider electrode E8 (curve71), which has its highest potential relative to electrode E4, and itslowest potential relative to electrodes E1 and E2, and an intermediatepotential relative to electrode E3. This corresponds to the actualelectrode positions shown in FIG. 7, where electrode E8 is closest toelectrode E4, somewhat further from electrode E3, and farthest fromelectrodes E2 and E1. A similar analysis for the monopolar stimulationfields of the other electrodes reveals a similar relationship: theelectrodes closest to the source electrode have the higher potential.

The electric field potentials for the tripolar stimulation are plottedon the same chart in FIG. 9. Again, the vertical axis is millivolts. Asseen in FIG. 9, a better relative orientation can be obtained than canbe obtained with the monopolar stimulation. Those electrodes closest tothe cathode have a high potential while those electrodes closest to theanode have a lower potential relative to the electrodes further away.For example, consider curve 72, which shows the electric field potentialof the non-active electrodes relative to the tripolar stimulation ofelectrodes E2 E3 E4, with E2 and E4 being anodes, and E3 being acathode. As seen in FIG. 9, curve 72 has a peak corresponding toelectrode E7, which means electrode E7 is closest to the cathode E3.Curve 72 further has lows or valleys corresponding to electrodes E6 andE8, which means E6 and E8 are closest to anode electrodes E2 and E4. Theactual orientation of the electrodes shown in FIG. 7 reveals that E6 isclosest to E2, and E8 is closest to E4. Thus, it is seen that thoseelectrodes closest to the flanked cathodic electrode have a highpotential while those electrodes closest to the anodic electrodes, oneither side of the cathodic electrode, have a lower potential relativeto the electrodes further away.

Hence, it is seen that by measuring the potential field of thenon-active electrodes, when active electrodes are stimulated at constantcurrent levels, e.g., subperception levels, the relative orientation ofthe neurostimulation leads may be determined. Once known, the relativeorientation may be used to track lead migration, to setup stimulationconfigurations and parameters for nominal stimulation and/or navigation,and to automatically adjust stimulation energy to a previously-definedoptimal potential field in the case of lead migration or posturalchanges.

Next, with reference to FIG. 10, a flowchart is shown that illustratesthe main steps that may be used to carry out and apply the inventiondescribed above in connection with FIGS. 7–9. As seen in FIG. 10, afirst step involves applying suitable stimuli, e.g., subperceptionstimuli, to a selected group of electrodes (block 82). Such applicationof stimuli defines the activated electrodes. While the stimuli are beingapplied to the activated electrodes, the electric field at thenon-activated electrodes is measured (block 84) and saved (block 86) aselectric field data (Block 86). Then, a determination is made as towhether there are other groups of electrodes that should be the“activated” electrodes, so that additional electric field measurementscan be made of the “non-activated” electrodes (block 85). If YES (thereare more electrode groups), then the steps shown at blocks 82, 84, and86 are repeated using the new group of activated electrodes. If NO (allthe electrode groups have been used), then the electric field data justobtained is compared to previously-saved electric field data for thesame non-activated electrodes (block 88).

The previously-saved electric field data may have been obtained duringinitial implantation of the leads, or during the last visit (severalweeks or months ago) to the doctor. Or, the previously-saved electricfield data may have been obtained just a new hours or minutes ago at atime when the patient's body had assumed a different posture position.Regardless of when the previously-saved electric field data wasobtained, the purpose of the comparison performed at block 88 of FIG. 10is to determine if the relative position of the leads has changed, whichchange in position would also have caused a relative change in theposition of the electrodes carried on the leads. Such determination maybe made by analyzing the electric field data (block 90) as describedabove in connection with FIG. 8 and/or FIG. 9 to determine whether therelative electrode orientation has changed.

The magnitude of the difference in the compared electric field data mayadvantageously provide a relative measure of how far the lead hasshifted or moved since the last electric field data was obtained.Advantageously, once a determination has been made that the leads(electrodes) have shifted relative to each other, appropriate correctionaction may be taken, as needed (block 92).

The corrective action taken at block 92 of FIG. 10 may include, forexample, simply tracking the lead migration over time, so that othercorrective action, e.g., surgery to reposition the leads, can be takenwhen necessary. Even if new surgery to reposition the leads is notneeded, simply mapping the lead migration over time will enablereprogramming of the stimuli parameters as needed so that a desiredeffect can be obtained regardless of the lead movement.

The corrective action may further include setting up stimulationconfigurations and parameters for providing nominal stimulation suitablefor the electrodes in their new relative positions. For example, theamplitude of the stimulus applied to one electrode may be decreased ifit is determined that the electrode has migrated closer to anotherstimulating electrode of the same polarity during stimulation, therebypreserving approximately the same stimulation effect for the patient.Alternatively, the amplitude of the stimulus applied to the electrodemay be increased if the electrode has migrated closer to a stimulatingelectrode of the opposite polarity. Such amplitude adjustments may bemade manually or automatically, depending on the mode of operation ofthe neurostimulation system.

Yet another corrective action that may be taken at block 92 of FIG. 10is to adjust the distribution of the stimuli to a new location throughnavigation. Navigation, as described in the previously referenced patentdocuments, involves electronically shifting the stimulus current fromone group of electrodes to another so as to shift or move the locationwhere the patient feels the most beneficial paresthesia, and/or receivesthe most benefit. Such navigation allows the neurostimulation system tobe quickly “fitted” to a given patient. Fitting the neurostimulationsystem to the patient is necessary after the system is first implanted,and may also be necessary whenever the leads (electrodes) have moved.The present invention thus provides a relatively easy way to determinewhether such lead movement has occurred, and thereby whether a refittingis or may be necessary.

Yet additional corrective action that may be taken at block 92 of FIG.10 in response to a determination that lead migration or posturalchanges have occurred includes manually or automatically adjusting thestimulation energy to a previously-defined optimal potential field.

It is thus seen that the present invention uses a measure of impedanceor electric field to determine relative lead positions for multipolarleads in a multi-lead configuration of a neurostimulation system, e.g.,a spinal cord stimulation system.

It is also seen that the invention uses impedance or electric fieldmeasurements to determine relative lead positions, which impedance orelectric field measurements may be used as an automated or assistivemethod for setting up a programmer for navigation, other programming, ordiagnostic evaluations in spinal cord (or other neural) stimulation.

It is additionally seen that the invention may be directed to thestoring of impedance or electric field maps to chronically trackrelative lead positions in a programmer linked to a database, along withother patient data.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

1. A method for determining the relative lead position of multipolarleads in a multi-lead configuration comprising measuring impedancevectors between electrodes; and inferring the relative position of thelead by the relative minima and maxima of the impedance values.
 2. Themethod of claim 1 wherein measuring the impedance vectors comprises:measuring the monopolar impedances for all electrodes; measuring bipolarimpedances between each electrode on opposing leads; and correcting thebipolar impedances for adjacent tissue impedance.
 3. The method of claim2 further including mapping the corrected bipolar impedances to createan impedance map, which impedance map reveals the relative orientationof the leads.
 4. The method of claim 3 further including storing the mapfor future reference.
 5. The method of claim 4 further includingreferring to the map at a future time to see if the lead position haschanged, and if so, how much.
 6. A method for determining whether therelative position of electrodes used by a neurostimulation system haschanged, the method comprising steps for: (a) applying a stimulus to aselected group of the electrodes; (b) measuring the electric field atthe electrodes not included in the group of electrodes to which thestimulus is applied; (c) repeating steps (a) and (b) over time anddetermining if changes in the measured electric field have occurred; (d)analyzing the changes in the electric field to determine whether changesin the relative position of the electrodes has occurred.
 7. The methodof claim 6 wherein step (a) comprises applying a stimulus monopolarlybetween one selected electrode and a return or reference electrode. 8.The method of claim 6 wherein step (a) comprises applying a stimulustripolarly between three selected electrodes, wherein at least one ofthe three electrodes comprises an anode, and at least one of the threeelectrodes comprises a cathode.
 9. The method of claim 8 wherein atleast two of the three electrodes are configured as an anode, and one ofthe three electrodes is configured as a cathode.
 10. The method of claim9 further including flanking the electrode configured as a cathode withthe electrodes configured as an anode, thereby creating a guardedcathode bipolar electrode configuration through which the stimulus isapplied.
 11. The method of claim 6 further including storing theelectric field measured at the electrodes not included in the group ofelectrodes through which the stimulus is applied.
 12. The method ofclaim 6 wherein step (c) includes plotting the measured electric fieldas a function of the selected electrode groups through which thestimulus is applied.
 13. The method of claim 6 further includinginitiating corrective action when a determination is made that therelative position of the electrodes has changed.
 14. The method of claim13 further including initiating the corrective action automatically. 15.The method of claim 13 wherein the corrective action includes adjustingthe amplitude of the stimulus applied to selected electrodes duringoperation of the neurostimulation system.
 16. The method of claim 13wherein the corrective action includes changing the electrodes throughwhich a stimulus is applied during operation of the neurostimulationsystem.
 17. The method of claim 13 wherein the corrective actionincludes adjusting the stimulation energy produced by theneurostimulation system to a previously-defined optimal potential field.18. The method of claim 13 wherein the corrective action comprisessetting up stimulation configurations and parameters that providenominal stimulation suitable for the electrodes in their new relativepositions.
 19. A method for determining the relative position ofelectrodes used by a neurostimulation system, the method comprisingsteps for: (a) applying a stimulus to a selected group of theelectrodes; (b) measuring the electric field potential at the electrodesnot included in the group of electrodes to which the stimulus isapplied; (c) analyzing the measured electric field potential at eachelectrode relative to the measured electric field potential at otherelectrodes to determine the position of the electrodes relative to eachother.
 20. The method of claim 19 wherein step (a) comprises applying astimulus monopolarly between one selected electrode and a return orreference electrode.
 21. The method of claim 19 wherein step (a)comprises applying a stimulus tripolarly between three selectedelectrodes, wherein at least one of the three electrodes comprises ananode, and at least one of the three electrodes comprises a cathode. 22.The method of claim 21 further including flanking the electrodeconfigured as a cathode with electrodes configured as an anode, therebycreating a guarded cathode tripolar electrode configuration throughwhich the stimulus is applied.
 23. The method of claim 19 wherein step(c) includes plotting the measured electric field as a function of theselected electrode groups through which the stimulus is applied.
 24. Anapparatus for determining the relative position or orientation ofelectrodes carried on neurostimulation leads, comprising: a multiplicityof implantable electrodes; means for applying a stimulus to a selectedgroup of the multiplicity of electrodes; means for measuring theelectric field at the electrodes not included in the group of electrodesto which the stimulus is applied; means for analyzing the measuredelectric field to determine the relative position or orientation of theelectrodes; and means for automatically initiating corrective action inthe event that a changes in the relative position or orientation of theleads has occurred.
 25. The apparatus of claim 24 further including:means for determining if changes in the measured electric field haveoccurred over time; and means for analyzing whether the changes thathave occurred in the electric field over time are indicative of a changein the relative position or orientation of the electrodes.
 26. Theapparatus of claim 24 wherein means for automatically initiatingcorrective action includes tracking the electrode orientation todetermine appropriate reprogramming.
 27. The apparatus of claim 26wherein appropriate reprogramming involves a setup of navigation tablesfor steering the multiple electrodes.
 28. The apparatus of claim 24wherein means for automatically initiating corrective action includestracking the electrode orientation to determine a need for surgicalrevision.